Conventionally, various displays including liquid-crystal displays use surface light source devices as backlights, front-lights and the like. For liquid-crystal displays featured small in thickness and light in weight, there are increasing demands for the displays of personal computers, etc. However, no light emissions occur on the liquid-crystal panel per se, external light or a supplementary light source is required in order to view the content being displayed. Meanwhile, there are recent needs toward thickness reduction and power saving due to greatly increasing demands for liquid-crystal displays on the mobile devices such as cellular phones and PDAs. For this reason, efficient utilization of light is of a problem of importance.
Under the technical background like the above, transition is gradually from those using so-called linear light sources, such as cold cathode tubes to surface light sources using LEDs (light-emitting diodes). There is a desire to construct a surface light source device with using LEDs in the number to a possible small extent.
The surface light source device using so-called a point light source is based on the principle that the light emitted from a point light source is introduced to the interior of a light-conductor plate through its light incident surface, and spread throughout the light-conductor plate while being totally reflected at between the main and back surfaces of the light-conductor plate. By a polarization pattern formed in the backside of the light-conductor plate, the light is caused to exit toward the front so that the light can exit at the surface of the light-conductor plate.
FIG. 1(a) is a plan view showing a structure of a conventional surface light source device 1, FIG. 1(b) is a cross-sectional view on line X-X in FIG. 1(a), and FIG. 1(c) is a bottom view of the surface light source device 1. In the surface light source device 1, a point light source 2 resin-encapsulated with an LED chip is mounted on a flexible printed board 4 by use of solder 3. The flexible printed board 4 is opened with a crimp hole 5. The light-conductor plate 6, a transparent resin mold, is formed with a light-emission window 7 in a corner thereof. In the vicinity of the light-emission window 7, a crimp pin 8 projects in an underside of the light-conductor plate 6. The point light source 2 is inserted in the light-emission window 7 of the light-conductor plate 6, and the crimp pin 8 is inserted through the crimp hole 5 of the flexible printed board 4. The crimp pin 8 is heated and crushed thereby effecting thermal crimping and fixing the point light source 2 onto the light-conductor plate 6.
However, in the surface light source device 1 of such a structure, in case the light-emission window 7 of the light-conductor plate 6 and the point light source 2 are the just equal in size, there is a difficulty in automatically inserting the point light source 2 in the light-emission window 7. Thus, a slight clearance is provided to between the point light source 2 and the light-emission window 7. Consequently, in the state the point light source 2 is mounted, a gap occurs between a front surface (light-emission window) of the point light source 2 and a wall surface (light incident surface) of the light-emission window 7. As shown in FIG. 2, light leak readily occurs through the gap toward the main and back surfaces of the light-conductor plate 6, thus making it impossible to efficiently introduce the light emitted from the point light source 2 into the light-conductor plate 6. Thus, there is a problem of lowering in light utilization efficiency. Particularly, on the surface light source device using a point light source 2, such light leak is to cause a great reduction of light utilization efficiency because the emission light amount at light source is lower as compared to the cold cathode tube or the like.
Meanwhile, in such a surface light source device 1, the flexible printed board 4 is thermally crimped by the crimp pin 8, the crimp pin 8 is considerably long in projection length even after thermal crimping if considering a setup strength. Usually, the thickness d in the crimping portion is as great as 0.3 mm. Consequently, the thickness of the surface light source device 1 overall including the crimp pin 8 is significantly great as compared to the thickness of the light-conductor plate 6 itself in the region other than the crimp pin 8, resulting in an increased thickness of the entire surface light source device 1 and hence a difficulty in reducing the thickness thereof.
Furthermore, in such a surface light source device 1, because the point light source 2 in plan is aligned with the light-emission window 7, the positional accuracy in plan thereof is favorable. However, in the direction thicknesswise of the light-conductor plate 6, the point light source 2 is aligned by abutting the flexible printed board 4 against an underside of the light-conductor plate 6. There undergo affections due to the variation, etc. in thickness of a solder 3 and inclination of an LED chip mounted, resulting in deviation in positional accuracy of the point light source 2 in the height direction. In case such variation in the height direction exists where a gap like the above is occurring between the front surface of the point light source 2 and the wall surface of the light-emission window 7 as mentioned above, light leak increases thus resulting in a problem of further lowering in light utilization efficiency.
For this reason, in order to eliminate the gap of between the front surface of the point light source 2 and the wall surface of the light-emission window 7, there is a proposal that a projection 9 protruding from the wall surface of the light-emission window 7 is abutted against a backside of the point light source 2 to thereby urge the front surface of the point light source 2 on the wall surface of the light-emission window 7. This structure can decrease the leak of the light emitted from the point light source 2. However, even this structure still requires a crimp pin 8 thus not solving the problem that the surface light source device 1 is increased in thickness into a difficulty to reduce the thickness.
Meanwhile, there are strong demands for device thickness reduction as seen in portable appliances, which inevitably imposes the need to reduce the thickness of the surface light source devices correspondingly. The conventional surface light source device has the overall thickness of 1.3 mm. Recently, demand is for a surface light source device having a thickness of 1.0 mm or smaller even at its point light source portion. Subtracting a flexible printed board thickness of 0.2 mm therefrom, the point light source must be suppressed 0.8 mm or smaller in thickness. There are a variety of proposals on the thickness reduction for the surface light source device. However, in the method of thermal crimping by placing the point light source 2 within the light-conductor plate 6 as in the surface light source device 1 shown in FIG. 1, the region crimped is as thick as 0.3 mm. Excepting the thickness of the crimping region and the thickness of the flexible printed board 4, there is a need for a point light source to have a thickness of 0.5 mm or smaller. At present, the point light source of side emission is minimally 0.6 mm thick.
In a surface light source device 10 shown in FIG. 4, a recess 11 is formed in an outer peripheral surface of a light-conductor plate 6, to arrange a point light source 2 in a manner opposing to the recess 11. A reflective sheet 12 is bonded between an upper surface of the point light source 2 and the upper surface of the light-conductor plate 6 while a reflective sheet 12 is bonded also between the lower surface of the point light source 2 and the lower surface of the light-conductor plate 6.
With this method, the light, emitted from the point light source 2 to the above of the surface of the light-conductor plate 6 or to the below of the back surface of the light-conductor plate 6, can be reflected upon the reflective sheet 12 and guided to the light-conductor plate 6, as shown in FIG. 5, which improves light utilization efficiency. However, in this method, the point light source 2 and light-conductor plate 6 are not sufficiently aligned before bonding the reflective sheet 12. In case variations occur in the gap between the point light source 2 and the light-conductor plate 6 due to the variation in bonding the reflective sheet 12, there is a decrease in the light introduced into the light-conductor plate 6 thus lowering light utilization efficiency. Furthermore, because the reflective sheet 12 is bonded on the both surfaces of the light-conductor plate 6, the surface light source device 10 increases in thickness. In addition, components increase in the number.
Meanwhile, in a surface light source device 17 shown in FIG. 6, snap-formed fitting holes 19 partly cut away are previously provided in a flange 18 formed of an exterior resin of a point light source 2. As shown in FIG. 7, the flange 18 with the point light source 2 is fitted over the outer peripheral surface of the light-conductor plate 6 in a manner covering a region a recess 11 is formed. By elastically fitting the fitting holes 19 of the point light source 2 with the fitting pins 20 projecting on the both surfaces of the light-conductor plate 6, the point light source 2 is fixed. However, even on this surface light source device 17, where there occur variations in the gaps between the point light source 2 and the light-conductor plate 6 due to errors in the fitting holes 19 or fitting pins 20, the light introduced to the light-conductor plate decreases to lower the light utilization efficiency. Furthermore, because the flange 18 is laid over the both surfaces of the light-conductor plate 6, the surface light source device 17 increases in thickness.
Meanwhile, in a surface light source device 21 shown in FIG. 8, crimp holes 22 are previously opened in both sides of a point light source 2 as shown in FIG. 9. After inserting the crimp pins 23 projecting in an outer peripheral surface of the light-conductor plate 6 into the crimp holes 22 of the point light source 2, the crimp pins 23 are thermally crimped to thereby attach the point light source 2 to the light-conductor plate 6. However, in this surface light source device 21, a gap readily occurs at between the point light source 2 and the outer peripheral surface of the light-conductor plate 6 due to variations in thermal crimping. There is a problem that, if a gap occurs, light leaks through it thus lowering light utilization efficiency.
With the surface light source device of a structure attaching a point light source on the outer peripheral surface of light-conductor plate as above, there is a fear to cause a gap at between the point light source and the light-conductor plate, thus involving a problem that light leaks through the gap thus worsen the light utilization efficiency and wherein the structure is not resistive to impacts.